The hardest part of the bench is getting it off your chest, not locking it out, so no, it isn't the most important factor.

Think about this. Who's arms will be lower when the bar is on the chest. The guy with the shorter arms or the longer arms? Think elbow angle. A short armed lifter may have more of a right angle to the arm whereas the long armed lifter will be pushing from much deeper. That alone makes it harder for the long armed lifter to get the bar off the chest. It's the same idea when people bench without going down all the way to the chest. They want to lift more so they shorten the movement. I do agree that the top of the lift doesn't really matter though.